This invention relates to medical electrical stimulation electrodes in general and to defibrillation electrodes in particular.
In the past years, there has been substantial activity toward development of a practical implantable defibrillator. Most proposals involve the use of large surface area implantable electrodes either to be mounted within the heart, on the exterior of the heart or subcutaneously. One common approach of providing a large surface area electrode is to employ an elongated exposed coil of biocompatible metal. In the context of an endocardial lead, this is disclosed in U.S. Pat. No. 4,161,952 issued to Kinney. In the context of an epicardial lead, this is disclosed in the context of U.S. Pat. No. 4,817,634 issued to Holleman et al.
In an epicardial lead, an elongated coil serving as the electrode can be mounted around the exterior of an insulative lead body. It is believed desirable in this context to stabilize the electrode coil with respect to the lead body, both to provide mechanical integrity and to prevent fibrous ingrowth around the individual coils of the electrode coil. In the above cited Kinney et al. patent, this is accomplished by sliding the coil over the lead body and backfilling the spaces between the electrode coil with a plastic material. The exterior surface of the electrode is then machined to provide a smooth surface. Alternatively, the backfilling may be removed by means of a plasma etch as disclosed in commonly assigned U.S. Pat. No. 4,934,049, issued to Keikhafer et al., for a "Method for Fabrication of a Medical Electrode". In this application, the backfilling is illustrated as extending radially outward between the turns of the coil about one-third to one-half the diameter of the coil wire. This patent is incorporated herein by reference in its entirety.
In prior U.S. Pat. No. 5,042,143 issued to Holleman et al., an alternative method of producing a lead structure similar to that produced in the Keikhafer patent, but without the necessity of using a backfilling, to produce a defibrillation lead employing polyurethane to stabilize the electrode coil and to fill between the turns of the coil. In the Holleman et al. patent, a plastic tube is stretched so that it displays an inner and outer diameter less than the inner and outer diameter of the tube in a relaxed state. An electrode coil having a inner diameter less than the outer diameter of the tube in its relaxed state is then slide over the stretched tube, after which the tube, after which the tube is released, allowing it to return to its previous length. However, the tube remains in a compressed state, throughout the length of the electrode coil, Thereafter, a mandrel having an outer diameter greater than the inner diameter of the tubing in its compressed state is passed into the tubing, to compress the tubing between the mandrel and the conductor coil. The assembly is thereafter heated, allowing the tubing to flow into spaces between the electrode coil.